Embodiments of the invention relate generally to x-ray tubes and, more particularly, to a liquid bearing assembly useable therewith and a method of bearing construction.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. The anode structure is typically supported by ball bearings and is rotated for the purpose of distributing the heat generated at a focal spot. Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed, which places stringent demands on the ball bearings.
A liquid lubricated or liquid metal bearing may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure at operating temperatures to meet the rigorous high vacuum requirements of an x-ray tube.
Liquid metals tend to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable for the bearing components. As such, a refractory metal such as molybdenum or tungsten is typically used as the base material for a liquid metal bearing. Not only are such materials resistant to corrosion, but they tend to be vacuum-compatible and thus lend themselves to an x-ray tube application. However, one concern that may be encountered in the use of a liquid metal is ensuring adequate wettability of bearing surfaces with the liquid metal. When adequate wettability does not occur, the liquid metal does not completely fill or lubricate the bearing and the liquid metal bearing may run out of liquid metal during use, thus shortening the life of the x-ray tube.
Liquid lubricated bearings are also highly sensitive to takeoff and landing events, which result in galling and wear on the bearing components and often constitute the failure mode of the bearing. To improve wear resistance and bearing performance a wear-resistant layer may be may be applied either to the moving or stationary surfaces of the bearing. The wear-resistant layers may, or may not, have poor wettability. If either the stationary or rotating surface is anti-wetting, then the bearing is considered “half-wetted”. In these “half-wetted” bearings, the lubricant and the solid surfaces of the rotating and stationary components of bearing assembly are selected so the lubricant wets the surface of one of the bearing components, thus forming a no-slip boundary with the surface of one bearing component, and does not wet the opposing surface and thus can slip against the surface of other bearing component.
While a “half-wetted” bearing design may mitigate galling and wear between bearing components, “half-wetted” bearings experience reduced load capability and lubricant containment issues due to the interaction between the wetted and non-wetted surfaces of the bearing components.
Accordingly, it would be advantageous to have an apparatus and method that improves lubricant containment, mitigates galling, and improves wear resistance of liquid lubricated bearings. It would further be desirable to reduce net costs associated with fabricating a liquid lubricated bearing.